1. Field of the Invention
The present invention relates to polishing, including chemical-mechanical polishing (CMP). In particular, the present invention relates to a mechanism for conditioning the surface of a polishing pad used in polishing operations.
2. Discussion of the Related Art
In sub-micron integrated circuits, CMP techniques are used to create the planarity required in multilevel interconnect structures. Specifically, to create a planar surface for depositing an interconnect layer, e.g. aluminum or titanium-tungsten, an interlayer dielectric (e.g., silicon dioxide) is planarized by a polishing process which uses a slightly alkaline colloidal slurry as a hydrolizing fine abrasive. One example of such a slurry includes fine silicon dioxide particles (e.g., average diameter of 70 nm) suspended in deionized water having an adjusted pH of approximately 11. The alkalinity can be provided by potassium hydroxide (KOH) and ammonium hydroxide (NH.sub.3 OH).
To maintain uniformity in the resulting surface of the interlayer dielectric and to provide reproducibility of the polishing process, the polishing surface, which is typically a polyurethane pad, is required to be conditioned between or during use. Conditioning is necessary to maintain the polishing pad to a uniform, textured or profiled surface.
FIG. 1 illustrates pad conditioning in the prior art. FIG. 1 shows, schematically, a prior art CMP apparatus 100. As shown in FIG. 1, CMP apparatus 100 includes a rotating platen 103, rotating in the direction indicated by reference numeral 105. On platen 103 is mounted a polishing pad 104. A silicon wafer (not shown) is held by a rotating polishing head 101 and pressed against the surface of polishing pad 104. Polishing head 101 rotates in a direction 109, generally in the same direction 105 of rotating platen 103. In addition, an oscillating arm 106 moves polishing head 101 to and fro along an arc indicated by reference numerals 108a and 108b. Correspondingly, a conditioning pad (not shown) is held by a smaller platen 102 against polishing pad 104. Platen 102 rotates in the direction indicated by reference numeral 110 and is moved to and fro along an arc indicated by reference numerals 107a and 107b by an oscillating arm 111. In this configuration, polishing pad 104 is continuously being conditioned in CMP apparatus 100 as a result of the motion in oscillating arm 111 and platen 102.
However, the conditioning process described in conjunction with FIG. 1 has at least one drawback. Specifically, the complex non-linear motions of the various components of CMP apparatus 100 often lead to excessive wear near the center of platen 103 and less wear in the periphery. Consequently, non-uniformity is introduced through polishing pad 104 into the wafer being polished.